What do I need?

Get a good sub and run it either off pre outs or speaker level, whichever you system dictates and be done. 
 

I have been running 2.1 so to speak for 20 years in various systems and it has always sounded great. 

Look into buying a used Classe SSP-800 AV processor. It has an LFE out for a sub and you're good to go. Does your preamp have Home Cinema Pass through? 

I have a denon avr with 15 channel s these days they have atmosphere witch puts front high middle high and rear high .this is so when things in the movies rly overhead they go from front to back..some music is now in atmos.ir your preamp has home theater pass through it allows the two front channels to be driven by your current amp.home theater can go from mild like stuff at best buy or separate home theater with separate multichannel processors with 30 separate monoamps for 15 channels.this is why mcintosh made the mi 502 class d amp for home theater.it does sound good in stero as well. You might have to read some home theater articles..personally I like my denon avr with 15 channels.its hard to go back to the theater.enjoy your search.and the music.

@tochsii 

What is the point of asking a question, if you aren't going to respond to the answers?

You wouldn't do this in a face to face conversation, so please respect the fact that the responses are offered to try and help you.

@tochsii 

Vinylshadow is right. Look for a used Classe SSP 800 av preamp surround processor in used markets. The SSP 800 originally retailed for $9500 but it’s been discontinued since 2017 and nowadays a used one can be had for around $2k. The Classe also works as a high end stereo pre as well and it sounds very very good in two channel for music. I would say in two channel sound quality wise the Classe SSP 800 is comparable to high end stereo preamps costing around $5k to $6k. Classe put priority and optimized the SSP 800 for music (two channel). 
What’s your stereo preamp? 
 

I happen to have the Classe SSP 800 up for sale on US AudioMart under caphill. Contact me on US AudioMart if you are interested. I’m selling it for $2k and is in mint condition and I have all original accessories : remote, packaging / box, user manual, etc.